Electric power assisted steering

Abstract

A combined motor and gearbox assembly for an electric power steering system comprising an electric motor, a gearbox which includes a gearset that is connected on an input part to a rotor of the motor and in use is connected on an output part at least indirectly to a steering column shaft to transfer torque from the motor rotor to the steering column shaft, a motor drive circuit, a gearbox housing which in use accommodates the gearset of the gearbox, the electric motor including a motor housing, and a baseplate which connects an end face of the gearbox housing to an end face of the gearbox housing and which supports the drive circuit. At least one bolt having a stem passes through a central hole in a tubular rivet that secures the motor housing to the baseplate, the bolt securing the baseplate to the gearbox housing.

Claims

1. A combined motor and gearbox assembly for an electric power steering system comprising: an electric motor, a gearbox which includes a gearset that is connected on an input part to a rotor of the motor and in use is connected at an output part at least indirectly to a steering column shaft to transfer torque from the motor rotor to the steering column shaft, a motor drive circuit, a gearbox housing which in use accommodates the gearset of the gearbox, the electric motor including a motor housing, and a baseplate which connects an end face of the gearbox housing to an end face of the motor housing and which supports the drive circuit, characterised by further comprising: at least one tubular rivet having a central through hole that secures the motor housing to the baseplate, and At least one bolt having a stem that passes through the through hole in the tubular rivet and secures the baseplate to the gearbox housing; wherein the surface of the gearbox housing immediately surrounding the threaded bore is recessed relative to the surrounding surface to accommodate the thickness of the rivet tail.

2. The combined motor and gearbox assembly according to claim 1 which includes three tubular rivets that each connect the motor housing to the baseplate, and three bolts that each have a stem that passes through a respective hole in a respective hollow rivet to connect the motor housing and the baseplate to the gearbox.

3. The combined motor and gearbox assembly according to claim 1 in which each rivet comprises a flat headed tubular rivet having a hollow cylindrical shaft, an enlarged head at one end and a tail of deformed material at the other end.

4. The combined motor and gearbox assembly according to claim 3 in which the head is located on the side of the baseplate opposite to the gearbox housing.

5. The combined motor and gearbox assembly according to claim 3 in which the head and tail are both hollow allowing access to the bore through the shaft for insertion of the bolt.

6. The combined motor and gearbox assembly according to claim 5 in which the hollow shaft of the rivet passes through a complementary shaped hole in the motor housing and in the baseplate.

7. The combined motor and gearbox assembly according to claim 6 in which the two holes are cylindrical and aligned on a common axis, that is also aligned with the axis of the shaft of the rivet.

8. The combined motor and gearbox assembly according to claim 1 in which each bolt carries an external thread on at least an end portion of the shaft furthest from the head of the shaft, which thread is received in a complementary internally threaded hole in the gearbox housing.

9. The combined motor and gearbox assembly according to claim 1 in which the motor drive circuit comprises an electronic control unit (ECU) which may comprise at least one processing chip fixed to the baseplate.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) There will now be described by way of example only one embodiment of the present invention with reference to the accompanying drawings of which:

(2) FIG. 1 is an isometric view of an embodiment of an electric steering sub-assembly in accordance with the present invention;

(3) FIG. 2 is a view from above of the motor can during fixing to the baseplate prior to fixing to the gearbox housing;

(4) FIG. 3 is a detail view showing one hollow rivet connecting the motor can to the baseplate;

(5) FIG. 4 is view corresponding to the view of FIG. 2 once the motor can and baseplate have been bolted to the gearbox housing;

(6) FIG. 5 is a detail view corresponding to the view of FIG. 3 once the motor can and baseplate have been fixed to the gearbox housing; and

(7) FIG. 6 is a schematic view of a complete steering assembly incorporating the sub-assembly of FIG. 1.

DETAILED DESCRIPTION

(8) As shown in FIG. 1, an electric steering sub-assembly 1 comprises an electric motor 2 located within a housing 3 in the form of a cylindrical can which is open at one end. The motor 2 includes a rotor 4 (visible in the schematic drawing of FIG. 6). This rotor is accessible through an opening in the housing and in use is fixed to a wormshaft 5 of a gearbox. The motor 2 is fixed to a gearbox housing 6 of the gearbox which accommodates the Wormshaft and also a worm wheel 7 on an input part of the gearbox (shown in the schematic of FIG. 6). In this example, the wormwheel is also connected to a steering column shaft 8 on the output part of the gearbox so that turning the wormwheel turns the input part and the output shaft. The steering column shaft 8 in use connects a steering wheel 9 of a vehicle to the road wheels 10, 11, typically through a rack and pinion 12.

(9) The motor 2 may be of any type but is preferably a three phase permanent magnet motor. The motor 2 is driven by a motor drive circuit 13 which typically comprises a three phase bridge driver circuit. The motor drive circuit 13 applies voltages to each phase in response to a torque demand signal received from a control circuit 14. In this example, the control circuit is combined with the drive circuit to form a close coupled electrical circuit.

(10) To join the motor housing 3 to the gearbox housing 6 and to also support the motor drive circuit including a motor control circuit a baseplate 15 is provided. This is a generally flat plate which seals the open end face of the motor housing can 3, and also seals a facing opening of the gearbox housing 6. The plate 15 is oversized to allow the drive circuit and control circuit to be located on the plate 15 offset from the motor can.

(11) The baseplate 15 is sandwiched between the gearbox housing 6 and the motor can 3 and all three are fixed in place by three hollow tubular rivets 16 and three bolts 17, each bolt 17 passing through the hollow rivet 16 to engage the gearbox housing. The manner in which the motor housing 3, baseplate 15 and gearbox housing 6 are fixed together will can best be understood referring to FIGS. 2 to 5 which show a preferred sequence of assembly.

(12) In a first stage of assembly, shown in FIGS. 2 and 3, the motor can 3 is fixed to the baseplate 15 and the drive stage circuit 13 and control stage 14 circuit are also fixed to the baseplate 15. The motor can 3 has a flange 3a around the periphery of its open end, and at spaced locations around the flange 3a are three cylindrical walled holes 18. These are aligned with three corresponding through holes 19 in the baseplate 15 and the stem of a hollow cylindrical rivet 16 is located through these holes with the head 20 of the rivet pressed onto the face of the flange around the hole and the tail on the opposing face of the baseplate around the hole. The rivet 16 can be installed by a conventional pressing process with a forming tool. Thus connected these components form an electrical power pack. At this stage, the electrical connections between the motor and drive stage and control stage can be made, and a motor position sensor, if required, can be attached or calibrated.

(13) In a next stage, the baseplate 15 is offered up to the gearbox housing 6 to bring the motor rotor into engagement with the wormshaft 5 which is exposed on a side of the gearbox housing. In this position, the three holes 19 in the baseplate 15 are aligned with three threaded bores 21 in the gearbox housing spaced around the exposed end of the Wormshaft. A bolt 17 with a stem 17b having a complementary thread is then inserted through the hole in the rivet and threaded into the hole in the gearbox housing until the head 17a of the bolt 17 contacts down onto the head 20 of the rivet.

(14) When assembled in this manner, the space needed to fasten the motor can 3 to the base plate 15 and then to fasten to the gearbox housing 6 is reduced as the fastenings are in the same axial location. Without the use of the hollow rivet, additional room for locating the bolts offset from the fasteners that hold the motor can to the baseplate would be needed. Notably, once finally assembled the bolt takes the tensile and shear loads between the gearbox housing and the baseplate and motor can, so that the rivet provides only a secondary function in taking that load. The rivet therefore only needs to be strong enough to hold the motor can to the baseplate during the first stage of assembly.